1. Field of the Invention
The present invention relates to an encoder, and more particularly relates to an absolute encoder wherein an output of a rotary member is coded, so that an absolute position of the rotary member can be detected directly.
2. Description of the Prior Art:
A magnetic type absolute encoder is formed by combining a detector utilizing magneto-resistive elements (which will hereinafter be referred as MR elements) with a magnetic drum on which magnetic patterns of more than two rows are recorded magnetically as described in Japanese Patents Laid-Open Nos. 54-118259 and 62-83619.
In general, magnetic patterns of n tracks are necessary in order to obtain a resolution of 2.sup.n in such absolute encoder. Accordingly, as shown in FIGS. 23 and 24, a plurality of tracks 30 (six in this case) are disposed on an outer peripheral surface of a magnetic drum 10, informations of three bits are recorded on said tracks 30, and a magnetic sensor 20 having MR elements (R.sub.01, R.sub.02 - - - ) of a number corresponding to said three bits facing the magnetic drum 10, so that an absolute value can be outputted by combining signals from the plural MR elements.
Japanese Patent Laid-Open No. 2-24518 discloses an absolute encoder wherein absolute signals can be obtained from tracks having absolute patterns.
However, it is necessary to arrange parallely a plurality of tracks on the magnetic drum 10 in the axial direction thereof, as shown in FIG. 23, in order to increase the resolution of the absolute encoder of the prior art, such as an absolute encoder shown in Japanese Patent Laid-Open No. 62-88619, thereby causing the absolute encoder large in size.
Further, it is necessary to describe informations of 2.sup.n on one track in order to obtain a resolution of 2.sup.n, for example, in an absolute encoder using circulatory random sequence codes, so that the absolute encoder must be made precisely in dimension and becomes expensive.
In the conventional absolute position detecting method described above, a plurality of absolute patterns are recorded on the peripheral surface of the magnetic drum in the circumferential direction thereof and MR elements corresponding thereto are arranged. In such case, it is better to increase the resolution by shortening the recording pitch in consideration of the size. However, the shortening of the recording pitch has a limitation in consideration of the arrangement of the MR elements, and if the recording pitch is increased, the magnetic drum becomes large in size.
Further, as shown in FIG. 24, leakage magnetic fluxes of N, S shown by arrows on the magnetic drum are sensed by the MR elements to generate signals in the absolute encoder. Accordingly, if the recording pitch is increased, the width of the magnetic sensor 20 becomes large as shown in FIG. 25. Accordingly, as shown in FIG. 25, a distance .alpha. between an MR element at an end portion of the magnetic sensor 20 and the center of the magnetic drum 10 becomes larger than a distance d between an MR element at the center portion of the sensor 20 and the center of the magnetic drum 10. Experiments show that a good sensing property cannot be obtained, because the MR element at the end portion of the magnetic sensor 20 cannot sense enough, when the relation of the .alpha. and d is expressed by EQU .alpha.&gt;d+0.1d.